I'm not sure if I need to use the chain rule here or not. I saw a video on YouTube where someone found that the $\frac {dy}{dx}$ of $y=xz$ is: $$\frac {dy}{dx} = x\frac {dz}{dx} + z$$
So I feel like I am more on track with the second one. Could someone explain how to take this derivative?
EDIT - I'm being asked if $u(x)$, but I'm not sure. The original problem is a differential equation that I need to solve using substitution:
$$ ydx + (1+ye^x)dy = 0 $$
It was suggested that I use the substitution $u=e^{-x}/y^2$, so, after solving for $y$, I need to find its derivative. Hence this question. Does this help clarify things? Is $u(x)$?